Is there a way to optimized volumetric fog either by changing the settings or isolating it to some sort of bounding box with out loosing the look and feel I've managed to achieve?

Hi everyone. I have a question regarding the exponential height fog with the volumetric fog checked. I used it as a way to get convincing light diffusion that happens in murky bodies of water. However this seems to be quite expensive when it’s been calculated. I’ve tried using a particle sim on a mesh to localize the effect, however I’m loosing the convincing deep and light diffuse look. Is there a way to optimized volumetric fog either by changing the settings or isolating it to some sort of bounding box with out loosing the look and feel I’ve managed to achieve?

Thanks in advanced!